The present invention relates generally to a packet switching system and an adaptation processing apparatus therefor. More particularly, the invention is concerned with an ATM (Asynchronous Transfer Mode) switch for exchanging or switching packets, each of a fixed length, and an adaptation processing apparatus suited profitably for use in combination with the ATM switch.
At present, studies of a broad band ISDN (Integrated Services Digital Network) are in progress as the next generation ISDN under the presidency of the CCITT (International Telegraph and Telephone Consultative Committee). The broad band ISDN is intended to provide communication services at a transmission rate, for example, of 150 M bits/sec. According to the CCITT Recommendations I. 121 (so-called blue book), the ATM (Asynchronous Transfer Mode) switching system is mentioned as promising means for realizing the broad band ISDN mentioned above.
In the ISDN, there are made available two types of channels, i.e. an information channel (B- or H-channel) for transferring user's information and a signal channel (D-channel) for transferring control signals for setting up communication paths or routes.
In the ATM switching system, all the data of the signal and information channels are transmitted and received in the form of fixed-length packets referred to as "cells". By way of example, let's assume that an ATM subscriber's terminal transmits a frame (signal frame) 50 of the signal channel having a format illustrated in FIG. 22A of the accompanying drawings. In that case, the signal frame 50 is once disassembled into cells 51A to 51N each of a fixed length (this processing is referred to as the segmentation processing), whereon the cells are sent out onto the subscriber's line. At the ATM switch, the signal 50 is reconstituted from the cells 51A to 51N as received (this processing is referred to as the reassembling processing), which is then followed by execution of the intrinsic signal processing. On the contrary, when the signal frame is transmitted to an ATM subscriber's terminal from the ATM switch, the segmentation processing is executed at the ATM switch while the reassembling processing is performed at the subscriber's terminal. Similarly, in the case of transmission/reception between the ATM switches, the regeneration processing is performed by the sending switch with the reassembling processing being executed at the receiving switch. These segmentation/reassembly processings are defined as the functions of an adaptation layer ranking lower than the layer 2 according to the CCITT recommendations I. 121 (blue book). In the following description, the processing apparatus of the adaptation layer for executing processings, including the segmentation/reassembling processings, is referred to as "ADP processing part or apparatus".
Schemes for realizing or implementing the ADP processing apparatus are known. For example, in the Paper B-168 entitled "A Study on Cell Assembling and Disassembling Functions in ATM Networks" published in the National Convention of the Institute of Electronics, Information Communication Engineers of Japan held in Autumn, 1989 and the Technical Study Report SSE 89-105 entitled "A Study of ATM Adaptation Layer Functions" published by the abovementioned Institute are known disclosed systems.